A method of water flow erosion for marine gas hydrate exploitation

ABSTRACT

The present invention provides a method of water flow erosion for marine gas hydrate exploitation. Based on the characteristics of higher permeability around gas hydrate exploitation well, controlling the seawater flow process by the pressure difference between hydrate reservoir and gas hydrate exploitation well. And the chemical potential difference between hydrate phase and water phase is the main driving factor for promoting the hydrate decomposition. Meanwhile, the salinity will increase and then the phase equilibrium temperature of hydrate will increase during seawater flow process. The water flow erosion accelerates the heat and mass transfer in hydrate reservoir to promote the efficient and complete decomposition and collection of hydrate. And the method of water flow erosion can decrease the risk of the geographical destruction caused by the large pressure drop. The present invention also provides the combination modes of water flow erosion with depressurization, thermal injection and other methods.

TECHNICAL FIELD

The present invention provides a method of water flow erosion for marine gas hydrate exploitation, and belongs to the marine gas hydrate exploitation technology field.

BACKGROUND

Nowadays, with the approaching depletion of traditional fossil fuel, the development and utilization of new energy resources has become the focus of research in the world. Natural gas hydrates (NGHs) are a new, clean, huge reserves and effective energy source with great potential for exploitation and has been a focus of research worldwide. There are mainly four methods of NGH exploitation, as follows: (1) depressurization, (2) thermal stimulation, (3) CO₂ replacement, (4) inhibitor injection and their combines.

Depressurization, which needs special requirements on gas hydrate reservoir, is considered to be the most efficient method for natural gas hydrates (NGHs) exploitation. And the depressurization is economically feasible, only if the conditions of gas hydrate reservoir near the phase equilibrium boundary. However, the insufficient decomposition driving force in the later stage of depressurization, the damage of hydrate reservoir and the stability of marine environment caused by depressurization are the main issues should be solved. Thermal stimulation has not solved the problem of low thermal utilization efficiency, and this method can only be used for local heating. Thus, this method needs to be further improved. The effect of the inhibitor injection method on hydrate reservoir is slower than others, and it will cause a series of environmental issues. What's more, the cost of this method is much higher. So there have been little researches about the method of inhibitor injection. The CO₂ replacement with the lower efficient can't achieve the complete hydrate exploitation, and there will be a risk of leakage.

According to the defects of current exploitation method, the present invention provides a method of water flow erosion for marine gas hydrate exploitation. Meanwhile, in order to improve the efficient of hydrate exploitation and the stability of hydrate reservoir, the present invention also provides various combination modes of water flow erosion with depressurization, thermal stimulation and others.

SUMMARY

According to the defects of current exploitation technology, and based on the influence of water flow on the hydrate stable existing, the present invention provides a method of water flow erosion for marine gas hydrate exploitation, which the hydrate decomposition is induced by chemical potential difference between hydrate phase and water phase and the higher heat and mass transfer. The hydrate is decomposed by controlling the pressure between hydrate reservoir and gas hydrate exploitation well, and further promoting the seawater flow in hydrate reservoir. Based on the characteristics of high permeability around the exploitation well, the chemical potential difference between water molecules in water phase and hydrate phase is used to drive the decomposition of hydrate. Meanwhile, the salinity will increase and then the phase equilibrium temperature of hydrate will increase during seawater flow process. What's more, the water flow erosion accelerates the heat and mass transfer in hydrate reservoir to promote the efficient and complete decomposition and collection of hydrate. And the method of water flow erosion can decrease the risk of the geographical destruction induced by the large pressure drop, which will cause the natural disasters. The present invention is easy to be implemented and it's conducive to the large-scale commercial exploitation of gas hydrate. Meanwhile, in order to improve the efficient of hydrate exploitation and the stability of hydrate reservoir, the present invention also provides various combination modes of water flow erosion with depressurization, thermal stimulation and others.

The technical solution of the present invention is as follows:

A method of water flow erosion for marine gas hydrate exploitation, comprising the following steps:

(1) Drilling location selection: drilling multiple gas hydrate exploitation channels, there will be two pressure control channels and one gas collection channel for one set of gas hydrate exploitation channel; drilling multiple water storage channels between adjacent exploitation channels in gas hydrate reservoir to interlink the pressure control channel with gas collection channel, which is used to storage seawater.

(2) For each gas hydrate exploitation channel, the middle channel is chose to be the gas collection channel, which is used to control the pressure of gas hydrate reservoir and the pressure difference between different gas hydrate exploitation channels. The seawater flowing process is controlled by the pressure difference between different gas hydrate exploitation channels, and the hydrate decomposition is induced by chemical potential difference between hydrate phase and seawater phase, which is caused by seawater flow process; meanwhile, the salinity of gas hydrate reservoir will increase during the seawater flow process, then, further increase the phase equilibrium temperature to promote hydrate decomposition. What's more, the seawater flow process will increase the heat and mass transfer in gas hydrate reservoir, which can be efficient to prevent the hydrate regeneration and ice generation.

(3) Changing the pressure of gas collection channel, increase the pressure difference between adjacent exploitation channels to increase the seawater flow velocity in seawater storage channels, and then promoting parts of hydrate decomposition, increase the water and gas permeability in gas hydrate reservoir.

(4) Adjusting the pressure of gas collection channel to stabile the pressure difference between adjacent exploitation channels and to promote the stability of seawater flow process. The best seawater velocity is chose to promote the hydrate exploitation. And then, achieving the efficient and safe hydrate exploitation in gas hydrate reservoir.

(5) The CH₄ is collected in the outlet of gas collection channel, and the pressure of gas hydrate reservoir is controlled by the gas collection process. Then, the collected gas is stored and transported. The separated seawater from the gas collection channels is injected into the pressure control channel to be used to the secondary flow in gas hydrate reservoir.

In order to achieve the efficient and safe hydrate exploitation, the method of depressurization or thermal injection in the early stage is used to decompose gas hydrate; and the water flow erosion method is used in the later stage of hydrate exploitation; the combination of the water flow erosion method with depressurization and thermal injection is helpful to improve the permeability of the gas hydrate production channels, the periphery of seawater storage channels and the interior of gas hydrate reservoir. Then, further increase the flow ability of seawater in gas hydrate reservoir, and finally achieve the efficient and safe hydrate exploitation by water flow erosion. It is conductive to solve the issues of insufficient driving force in the later stage of depressurization and the low efficiency of thermal injection.

The most beneficial effect of the invent is proposed a method of water flow erosion for marine gas hydrate exploitation and achieve the effective combination of water flow erosion, depressurization and thermal injection, which can solve the defects of other hydrate exploitation methods and the difficulties in the actual production tests. In addition, the method of water flow erosion also can provide a feasible method for the large-scale exploitation of natural gas hydrate, and decrease the risk of geographical destruction induced by the large pressure drop, which will cause the natural disasters. Meanwhile, the water flow erosion has great significance for the further experiment investigation and actual hydrate production.

DESCRIPTION OF DRAWINGS

FIG. 1 is a principle diagram of the water flow erosion method.

FIG. 2 is a schematic diagram of the water flow erosion exploitation method.

FIG. 3 is a schematic diagram of the combination method of the water flow erosion with depressurization.

FIG. 4 is a schematic diagram of the combination method of the water flow erosion with thermal injection.

DETAILED DESCRIPTION

The FIG. 1 shows the principle diagram of the water flow erosion method for marine gas hydrate exploitation, in which the hydrogen bond rupture caused by the chemical potential difference between hydrate phase and water phase, and then the gas hydrate is decomposed and the gas molecule is released. What's more, the water flow erosion accelerates the heat and mass transfer in hydrate reservoir and promotes the hydrate decomposition in many respects.

Specific embodiments of the present invention are further described below in combination with accompanying drawings and the technical solution.

Embodiment 1

Firstly, drilling multiple gas hydrate exploitation channels using the low-density mud drilling technology, and the water flow erosion method will be conducted through these channels for marine gas hydrate exploitation. Take one set of exploitation channels to illustrate the hydrate exploitation process. One set of gas hydrate exploitation channel includes three channels: one gas collection channel in the middle and two pressure control channels on both sides.

As shown in FIG. 2, three suitable drilling locations are selected in the middle of the reservoir and three exploitation channels are drilled in the hydrate reservoir using the low-density mud drilling technology. And then, multiple horizontal water storage channels between adjacent exploitation channels in gas hydrate reservoir are drilled, which are used to storage seawater and facilitate the flow of seawater.

The middle channel is chose to be the gas collection channel and the adjacent channel is chose to be the pressure control channels. The pressure of gas hydrate reservoir and the pressure difference between different gas hydrate exploitation channels is adjusted by gas collection channel. The seawater flowing process in the horizontal water storage channels is controlled by the pressure difference, therefore, the recovery rate of gas hydrate exploitation can be controlled.

Changing the gas collection rate at the outlet of gas collection channel and then changing the pressure of gas collection channel, increase the pressure difference between adjacent exploitation channels to increase the seawater flow velocity in horizontal seawater storage channels. And then promoting parts of hydrate rapid decomposition to increase the water and gas permeability in gas hydrate reservoir. It is beneficial to control the velocity of seawater in the seawater storage channel.

In the process of natural gas hydrate exploitation, the pressure of hydrate reservoir is kept stable at the same as the pressure of the unexploited reservoir, which ensures the stability of the reservoir environment. In addition, it also decreases the risk of the geographical destruction induced by the large pressure drop, which will cause the natural disasters.

The pressure of gas collection channel is adjusted to stabile the optimal pressure difference between adjacent exploitation channels and to promote the stability of seawater flow process. The best seawater flow velocity is chose to promote the hydrate exploitation. And then, the efficient and safe hydrate exploitation in gas hydrate reservoir is achieved.

The CH₄ is produced by hydrate decomposed is collected in the outlet of gas collection channel, and the collected gas is stored and transported. The separated seawater from the gas collection channels is injected into the pressure control channel to be used to the secondary flow in gas hydrate reservoir.

Embodiment 2

The following is described in detail in combination with FIG. 3.

The combination mode of water flow erosion method with depressurization drills multiple gas hydrate exploitation channels utilizes the low-density mud drilling technology. At first, parts of hydrate are decomposed by depressurization, which increased the water permeability and mobility in gas hydrate reservoir. After that, the water flow erosion is used to exploit gas hydrate. And take one set of exploitation channels to illustrate the hydrate exploitation process. One set of gas hydrate exploitation channel includes two channels: pressure control and gas collection channel, pressure control and backflow channels.

As shown in FIG. 3, two suitable drilling locations are selected and two exploitation channels are drilled in the middle of the hydrate reservoir using the low-density mud drilling technology. And then, multiple horizontal water storage channels between and near the two exploitation channels in gas hydrate reservoir are drilled, which is used to storage seawater and facilitate the seawater flow.

One exploitation well is selected as the pressure control and gas collection channel, and the other exploitation well is chose to be the pressure control and backflow channels. At the early stage of hydrate exploitation, the depressurization method is used to decompose hydrate. Decrease the pressure of the pressure control and gas collection channel to promote parts of hydrate decomposition and to increase the water and gas permeability of exploitation channels, surrounding storage water channels and the hydrate reservoir, which can accelerate the water and gas mobility during the seawater flow process.

After parts of hydrate decomposition by depressurization, a large amount of gas is produced from the hydrate reservoir, which can be used to control the pressure difference between the two hydrate exploitation channels. And the pressure difference can promote the seawater flow between two exploitation channels. Therefore, the hydrate decomposition can be realized by controlling the seawater flow velocity, and further achieving the complete decomposition of residual hydrate by seawater flow erosion.

In the middle and late stage of hydrate exploitation process, combining the water flow erosion with depressurization, the hydrate decomposition rate can be controlled by adjusting the stable pressure difference between two exploitation channels and water flow velocity. It will avoid the geological instability induced by the massive hydrate decomposition in a short time and solve the issue of low production efficiency in the later stage of hydrate exploitation.

The CH₄ is produced by hydrate decomposed is collected in the outlet of gas collection channel, and the collected gas is stored and transported. The separated seawater from the gas collection channels is injected into the pressure control and backflow channel to be used to the secondary flow in gas hydrate reservoir.

Embodiment 3

The following is described in detail in combination with FIG. 4.

The combination mode of water flow erosion method with thermal injection for marine gas hydrate exploitation drills multiple gas hydrate exploitation channels utilizing the low-density mud drilling technology. At first, parts of hydrate are decomposed by thermal injection, which increased the water permeability and mobility in gas hydrate reservoir. After that, the water flow erosion is used to exploit gas hydrate. And take one set of exploitation channels to illustrate the hydrate exploitation process. One set of gas hydrate exploitation channel includes two channels: seawater injection well, gas collection well.

As shown in FIG. 4, two suitable drilling locations are selected and two exploitation channels are drilled in the middle of the hydrate reservoir using the low-density mud drilling technology. One of them is chose to be the seawater injection well and the other is chose to be the gas collection well.

The seawater is injected into the seawater injection well using the injection pump, and the seawater will flow to the surrounding hydrate reservoir from the seawater injection well. Due to the existence of temperature difference between seawater and gas hydrate reservoir, the heat of seawater will transfer to the surrounding hydrate reservoir during the flow process by heat convection and heat conduction. The higher temperature will promote the partial hydrate decomposition, which will open the gap channel and form a highly permeable area around the seawater injection well, where is conductive the seawater flow. The initial amount of seawater injection can be determined according to the exploitation parameters, such as seawater temperature, hydrate reservoir temperature, and hydrate saturation, layer permeability, heat conductivity coefficient, injection pump characteristics and so on.

A large amount of gas is produced from the hydrate reservoir, which can be used to control the pressure difference between the two hydrate exploitation channels. And the pressure difference can promote the seawater flow between two exploitation wells.

In order to achieve the efficient and safe hydrate exploitation in gas hydrate reservoir, the seawater flow velocity is controlled by controlling the pressure difference between two exploitation wells, and further realizing the stable hydrate decomposition rate.

The CH₄ is produced by hydrate decomposed is collected in the outlet of gas collection channel, and the collected gas is stored and transported. The separated seawater from the gas collection channels is injected into the pressure control and backflow channel to be used to the secondary flow in gas hydrate reservoir.

The above implementation plans are the three specific modes of the invention, the general changes and substitutions within the scope of the technical scheme should be included within the protection scope of the invention. 

We claims:
 1. A method of water flow erosion for marine gas hydrate exploitation, wherein it comprises the following steps: (1) drilling location selection: drilling multiple gas hydrate exploitation channels, there will be two pressure control channels and one gas collection channel for one set of gas hydrate exploitation channel; drilling multiple water storage channels between adjacent exploitation channels in gas hydrate reservoir to interlink the pressure control channel with gas collection channel, which is used to storage seawater; (2) for each gas hydrate exploitation channels, a middle channel is chose to be the gas collection channel, which is used to control the pressure of gas hydrate reservoir and the difference pressure between different gas hydrate exploitation channels; the seawater flowing process is controlled by the difference pressure between different gas hydrate exploitation channels, and the hydrate decomposition is induced by chemical potential difference between hydrate phase and seawater phase, which is caused by seawater flow process; meanwhile, the salinity of gas hydrate reservoir will increase during the seawater flow process, then, further increase the phase equilibrium temperature to promote hydrate decomposition; what's more, the seawater flow process will increase the heat and mass transfer in gas hydrate reservoir, which can be efficient to prevent the hydrate regeneration and ice generation; (3) changing the pressure of gas collection channel, increase the difference pressure between adjacent exploitation channels to increase the seawater We claims velocity in seawater storage channels, and then promoting parts of hydrate decomposition, increase the water and gas permeability in gas hydrate reservoir; (4) adjusting the pressure of gas collection channel to stabile the difference pressure between adjacent exploitation channels and to promote the stability of seawater flow process; the best seawater velocity is chose to promote the hydrate exploitation; and then, achieving the efficient and safe hydrate exploitation in gas hydrate reservoir; (5) the CH₄ is collected in the outlet of gas collection channel, and the pressure of gas hydrate reservoir is controlled by the gas collection process; then, the collected gas is stored and transported; the separated seawater from the gas collection channels is injected into the pressure control channel to be used to the secondary flow in gas hydrate reservoir.
 2. The method of water flow erosion for marine gas hydrate exploitation according to claim 1, is characterized in that the parts of gas hydrate is decomposed by the method of depressurization and thermal injection in the early stage to increase the local permeability of gas hydrate reservoir; and the method described in claim 1 will be used in the later stage of hydrate exploitation; the combination of the method described in claim 1 with depressurization and thermal injection is helpful to improve the permeability of the gas hydrate production channels, the periphery of seawater storage channels and the interior of gas hydrate reservoir; then, increase the flow ability of seawater in gas hydrate reservoir, and further achieve the efficient and safe hydrate exploitation by water flow erosion. 